the fight is on for aleppo, syria's largest city and one of the world's oldest. the rebels call it a decisive battle. it is also deadly. this weekend the intense combat triggered a massive fire in the city's ancient marketplace. a priceless landmark, reducing much of it to ashes. we made our way in to syria, past scenes of devastation, to the outskirts of aleppo. to meet rebel commander ahmed afaj once a local businessman now driven by the loss of his own brother, killed in combat. he leads more than 2500 rebels. all sunni muslims. and says he is battling shiite militants fighting for the regime, mostly from outside syria. iranian and hezbollah fighters are here in large numbers, he says. when we kill them, we find their i.d. cards. the u.s. has accused iran of meddling in syria. the commander goes farther. he says of the regime, all of the planes they're using, the new ones, are from iran. and, he says, government planes are dropping a new kind of weapon. bombs in barrels, with chemicals, he says, that take out city blocks. a whole family was in here, he says, a